1. Field of the Invention
The invention relates to organopolysiloxane-containing, water-based compositions which are essentially free from organic solvents, have a flash point of above 70.degree. C. and liberate essentially no alcohols through hydrolysis on dilution with water, to a process for their preparation, and to their use. The organopolysiloxane-containing compositions are essentially free from solvents. The term solvent-free compositions here is taken to mean those which contain no purely organic aliphatic or aromatic hydrocarbons, halogenated hydrocarbons, glycols, glycol ethers, ethers, ketones, esters, amides and other nitrogen-containing compounds, sulfur compounds and nitro compounds, and proton- (or hydrogen-) free solvents in amounts of greater than 100 ppm by weight. By contrast, proton-containing agents, such as water, inorganic or organic acids, alcohols, such as methanol, ethanol, n- and i-propanol, and higher alcohols in this series, are not regarded as organic solvents hereinafter.
2. Background of the Invention
Organosilanes of the general formula R.sup.0 --Si(OR.sup.1).sub.3, where R.sup.0 is an organic group and R.sup.1 is a methyl or ethyl group, have a wide variety of applications, for example as adhesion promoters, release agents, rheology improvers, crosslinking agents and hydrophobicizing agents.
For ecological, safety and economic reasons, there is an increasing trend to apply these compounds in aqueous form, which generally gives rise to the following problems:
Hydrolysis causes liberation of alcohols: EQU R.sup.0 --Si(OR.sup.1).sub.3 +3H.sub.2 O.fwdarw.R.sup.0 --Si(OH).sub.3 +3R.sup.1 OH
Free alcohols lower the flash point of the application solution, so that explosion-protected machines and special equipment have to be employed for the necessary processing steps. For toxicological reasons, the persons handling these compounds must additionally be trained and protected. Furthermore, disposal of the used application solution can cause difficulties due to the hydrolysis of alcohols. Special effluent treatment plants, as well as equipment for off gas and exhaust air treatment also are required.
The organosilane to be applied in water is not water-soluble: EQU R.sup.0 --Si(OR.sup.1).sub.3 +1.5H.sub.2 O.fwdarw.R.sup.0 --Si(OH).sub.1.5 +3R.sup.1 OH
The hydrolyzate R--SiO.sub.1.5 formed in accordance with this equation precipitates from the reaction mixture as a polymeric silicone resin, so that it is no longer available for the desired application. Although organosilanes having a highly hydrophobic character, for example alkylalkoxysilanes, in particular those having a long or branched carbon skeleton, hydrolyze only very slowly, they are insoluble in water.
Although monomeric organosilanes containing acrylic, methacrylic and glycidyl ether functional groups, for example methacryloxypropyltrimethoxysilane (DYNASYLAN.RTM. MEMO) or glycidyloxypropyltrimethoxysilane (DYNASYLANO.RTM. GLYMO) are substantially soluble in water, they tend to polycondense in aqueous solution and therefore do not have a long shelf life in aqueous solution. For example, GLYMO has a shelf life of about 10 days in water, while MEMO has a shelf life of only around 2 days (cf HUELS Product Information Sheet-. use of DYNASYLAN.RTM. organofunctional silanes, July 1994).
In order to obviate the known disadvantages described above, a wide variety of processes have been suggested in the past:
Some organosilanes, for example aminoalkylalkoxysilanes, while being stable in aqueous solution, have only restricted practicability in the wide range of possible uses of organofunctional silanes. The introduction of further organic functionalities, in particular a plurality of functionalities in one molecule based on organosilanes which are not stable per se in aqueous solution, is of considerable interest with regard to modification of the use properties.
A means for removing hydrolysis alcohols by distillation during the synthesis for the preparation of organosilane formulations from water-soluble organosilanes, for example from 3-aminopropyltriethoxysilane, is known. Water-based aminosilane systems which have a low alcohol content and otherwise contain no solvents are obtainable in this way and are commercially available, for example in the form of DYNASYLAN.RTM. 1151.
In the emulsion method, which likewise belongs to the prior art, the organosilane which is not compatible with water per se is emulsified in water with the aid of emulsifiers (EP A 0 442 098, European Patent 0 358 652 and U.S. Pat. No. 4,620,878). It is disadvantageous here that the products contain significant proportions of surfactants as emulsifiers and can liberate considerable amounts of alcohol.
The formulation of silane combinations in water is disclosed in U.S. Pat. No. 5,073,195. Silane formulations are prepared from a water-insoluble alkyltrialkoxysilane and a water-soluble silane, for example an aminoalkylalkoxysilane, in a molar ratio of from 1:2 to 3:1. As shown in the examples of this U.S. Patent, the formulations are prepared by substoichiometric hydrolysis of the silane mixture and stripping of the reaction mixture at 60.degree. C. under reduced pressure, and for subsequent applications, silane formulations obtained in this way are also diluted with water. However, the dilution with water results in elimination of the alkoxy groups remaining after incomplete hydrolysis in the form of the corresponding alcohols. The silane cohydrolyzates thus contain free alcohols in significant amounts and may in addition liberate further amounts of alcohol through hydrolysis, which adversely affects the use properties of the products.
European Patent Application EP 675 128 describes the modification of water-based organosilane formulations with small amounts of organosilanes which are not water-soluble per se, for example methyltrimethoxysilane or vinyltrimethoxysilane. This process allows a molar ratio between the water-insoluble component and the water-soluble component of 1:2.5 to be achieved. If the content of water-insoluble silanes is higher, highly viscous products which gel over time are formed. Use properties essentially correspond to those of the abovementioned water-based organosilane formulations. By this process aqueous solutions which contain higher proportions of water-insoluble organosilanes and are stable for months are not obtainable. The preparation of aqueous solutions containing high proportions of highly hydrophobic silanes, for example isobutyltrimethoxysilane, is not possible by this process.
EP 0 716 127 and EP 0 716 128 disclose the preparation of water-soluble, solvent-free and essentially hydrolysis alcohol-free organosilane formulations based on a water-soluble silane (aminosilane) serving as "carrier silane" with high proportions of water-insoluble silanes (alkylsilanes, vinylsilanes or ureidosilanes). The water solubility of formulations is achieved by targeted cohydrolysis of aminosilanes with water-insoluble silanes, followed by neutralization of the amino function by means of acids and distillation of hydrolysis alcohols. However, the adhesion-promoting properties of these systems are of only limited acceptability for some applications. In addition, there is a need for organosilane systems which, when applied to a surface to be silanized, usually a mineral one, produce both hydrophobic and adhesion-promoting effects at the same time. There is therefore also a need for water-based and essentially solvent-free organosilane formulations which have a low alcohol content, can be employed, for example, instead of hydrophobicizing silanes, such as alkylalkoxysilanes, and likewise exhibit both a hydrophobicizing and a satisfactory adhesion-promoting action on a mineral surface to be silanized.